Synthesis and structure of 4-hydroxy-N-isopropyltryptamine (4-HO-NiPT) and its precursors

The synthesis of the norpsilocin derivative, 4-hydroxy-N-isopropyltryptamine, is presented, as well as its crystal structure and the structures of its three synthetic precursors.


Chemical context
Psilocybin (C 12 H 17 N 2 O 4 P, 4-phosphoryloxy-N,N-dimethyltryptamine) has recently garnered a great deal of interest due to its potential to ameliorate a number of treatment resistant mood disorders (Carhart-Harris & Goodwin, 2017;Nichols et al., 2017). Upon administration, psilocybin is enzymatically metabolized via hydrolysis of its 4-phosphoryloxy group, producing psilocin (C 12 H 16 N 2 O, 4-hydroxy-N,N-dimethyltryptamine) as the active metabolite. Psilocin is an agonist of the serotonin 2A (5-HT 2A ) receptor; this activity is believed to be responsible for producing a head-twitch response (HTR) in murine models, as well as subjective 'psychedelic' effects in human subjects as well as other potentially beneficial biological and clinical results .
In the case of baeocystin, its active metabolite norpsilocin (4-hydroxy-N-methyltryptamine) has been examined and shown to be a full agonist of the 5-HT 2A receptor (Sherwood et al., 2020;Glatfelter et al., 2022b). Notably, despite this activity, norpsilocin does not show a head-twitch response (HTR) in mice, the standard animal test to indicate a psychedelic-like response. Unlike dialkyl tryptamines (e.g., psilocybin and psilocin) the pharmacology of analogous monoalkyl tryptamines (e.g., baeocystin and norpsilocin) is relatively unknown. Accordingly, the importance of these compounds within the context of the overall polypharmacological 'magic mushroom' experience is not understood. In an effort to explore the proprieties of monoalkyltryptamines, we previously reported the structural characterization, cellular, and behavioral data for baeocystin and norpsilocin (Naeem et al., 2022;Chadeayne et al., 2020b).
Herein, we expand our exploration to include other 4-hydroxy monoalkyl tryptamines in an effort to examine how the steric variation impacts serotonergic activity. Our first target, 4-hydroxy-N-isopropyltryptamine (4-HO-NiPT), replaces the methyl group of norpsilocin with an isopropyl group. The only previous literature report of this molecule is as a metabolite of the new psychoactive substance (NPS) 4-acetoxy-N,N-diisoproptyltryptamine (4-AcO-DiPT) from 2022 (Malaca et al., 2022). The synthesis of the title compound, 4-HO-NiPT, follows a procedure modified from the psilocin synthesis put forward by Albert Hofmann in 1959 (Troxler et al., 1959). The structure of 4-HO-NiPT and those of its three synthetic precursors are reported herein.

Structural commentary
The asymmetric unit of 4-benzyloxyindole, C 15 H 13 NO (1) contains a single molecule (Fig. 1). The indole ring system of the tryptamine grouping is almost planar with an r.m.s. deviation from planarity of 0.013 Å . The benzyloxy group has an anti conformation with a C6-O1-C9-C10 torsion angle of À179.00 (13) . The benzene ring of the benzyloxy group is near perpendicular from the indole ring with a plane to plane twist of 89.73 (6) .
The asymmetric unit of 4-hydroxy-N-isopropyltryptamine C 13 H 18 N 2 O (4) contains a single tryptamine molecule (Fig. 1). The indole ring system is almost planar with an r.m.s. deviation of 0.006 Å . The ethylamine arm of the tryptamine is turned away from the indole ring, with a C7-C8-C9-C10 torsion angle of 75.2 (2) whereas the C8-C9-C10-N2 torsion angle of À72.6 (2) turns the amine group back toward the hydroxide substituent on the 4-position of the indole ring system. The turn of the ethylamine arm is due to an intramolecular O-HÁ Á ÁN hydrogen bond between the hydroxide group and the amine N atom.

Supramolecular features
There are no significant intermolecular interactions in (1) beyond normal van der Waals contacts. The molecules of (2) are linked by N-HÁ Á ÁO hydrogen bonds, generating infinite chains along the [100] direction between indole N atoms and the O atoms of the amide carbonyl groups (Table 1). The tryptammonium cations and chloride anions of (3a) are linked into infinite chains propagating along [010] by N-HÁ Á ÁCl hydrogen bonds between the indole nitrogen atoms and the chloride anions and the ammonium N atoms and the Cl À ions ( Table 2). The tryptamine molecules of (4) are held together in infinite chains along the [010] direction by N-HÁ Á ÁO hydrogen bonds between the indole NH groupings and the hydroxide O atoms (Table 3). The crystal packing of compounds (1)-(4) are shown in Fig. 2.

Synthesis and crystallization
4-Benzyloxyindole (1): Single crystals of (1) were grown from the vapor diffusion of diethyl ether into a methylene chloride solution of a commercial sample (Biosynth).
N-isopropyl-4-benzyloxy-3-indoleglyoxylamide (2): to a solution of (1) (2.0 g, 8.96 mmol) in diethylether (50 ml) was added oxalylchloride (2.3 g, 17.93 mmol) dropwise at 273 K. The resulting mixture was stirred for 6 h at 273 K, and 2propylamine (4.24 g, 71.68 mmol) was added dropwise. The mixture was warmed to room temperature and stirred overnight. Solvent was removed in vacuo, and the resulting residue was purified on a silica gel column (methylene chloride/ methanol) to afford the product as a yellow oil (2.9 g, 96% yield). Single crystals of (2) suitable for X-ray diffraction studies were grown by vapor diffusion of diethyl ether into a methylene chloride solution.

Table 3
Hydrogen-bond geometry (Å , ) for (4). (15) 176 (2) H3A, H4A and H4B in compound (3a) and H1, H1A and H2 in compound (4) were found from difference-Fourier maps and were refined isotropically. DFIX restraints were used on all of these hydrogen atoms [except H1 in compound (1), which was refined freely] with N-H distances of 0.87 (1) Å for the indole N atoms, 0.90 (1) Å for the ethylamino N atoms, 0.95 (1) Å for ethylammonium N atoms, and 1.00 (1) Å for the O-H distance. Isotropic displacement parameters were set to 1.2 U eq of the indole N atoms and 1.5 U eq of the parent ethylamino N atoms and the parent oxygen atom. All other hydrogen atoms were placed in calculated positions (C-H = 0.93-0.98 Å ). Isotropic displacement parameters were set to 1.2 U eq (C) or 1.5 U eq (C-methyl). Table 4 Experimental details.

4-Phenoxy-1H-indole (1)
Crystal data  Extinction correction: SHELXL-2018/3 (Sheldrick 2015b), Fc * =kFc[1+0.001xFc 2 λ 3 /sin(2θ)] -1/4 Extinction coefficient: 0.0078 (11) Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. Refinement. Refined as a 2-component inversion twin.

Refinement
Refinement on F 2 Least-squares matrix: full R[F 2 > 2σ(F 2 )] = 0.045 wR(F 2 ) = 0.116 S = 1.08 2461 reflections 159 parameters 3 restraints Hydrogen site location: mixed H atoms treated by a mixture of independent and constrained refinement w = 1/[σ 2 (F o 2 ) + (0.0509P) 2 + 0.7218P] where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.18 e Å −3 Δρ min = −0.15 e Å −3 Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.